1. Field of the Invention
This invention relates to detectors used in time-of-flight mass-spectro-meters having some kind of ion-electron conversion surface.
Detectors for time-of-flight mass-spectrometers should oppose the in-coming beam with an aperture as large as possible and even with this large aperture they should cause as little timing errors as possible.
Every detector must have some kind of ion-electron conversion surface. At the instant that an ion impinges on that surface there is a certain probability that one or more electrons are created, which are amplified in electron-amplifiers. This amplification has as result an electrical impulse that gives information about the time-of-arrival of that ion.
As an alternative to electron-amplifiers a combination of scintillator and photomultiplier can also be used.
The ion optical axis is understood as one path, said path selected at or close to the center of the incoming ion beam. Should the detector have a construction of cylindrical symmetry, then usually the axis of symmetry is chosen.
Starting from the ion-electron conversion surface, one can follow the ion optical axis in reverse direction out of the detector up to a conveniently chosen point. Normal to the ion optical axis one can define a reference plane. As reference-time-of-flight one can define the time-of-flight from that reference plane onto the ion-electron conversion surface. If ions are started from the reference plane at other points than the axis point but with the same direction and velocity, these ions may need different flight times than an ion started on the axis point would need. The difference between these flight times and the reference-time-of-flight are called time errors.
These time errors can be given as a function of the starting location on the reference plane. In the most general case the time errors are a function of the two variables or parameters defining the reference plane. If the detector is constructed with rotational symmetry around a straight axis, the time errors are a function of the distance a path has from the ion optical axis in the reference plane.
Within a detector having inhomogeneous electrical fields ions can either he focused onto a smaller surface or defocused onto a larger surface. For that reason the usable surface on the ion-electron conversion surface is not a good measure for the sensitivity of the detector. As a measure of sensitivity one can use the size of that portion of the reference plane from which ions can be started with acceptably low timing errors into the detector.
By defining a reference plane and just considering the paths from the reference plane to the ion-electron conversion surface one can logically separate the detector and its timing errors from the rest of the time-of-flight mass-spectrometer. On the other hand, it is also possible to determine the timing errors of complete paths from the ion source to the conversion surface. Aside from timing errors that result directly from the detector and its construction, the paths may have timing errors in the ion source and the reflector that can be compensated by tilting the ion-electron conversion surface. For that reason the conversion surface is often supported such that its orientation can he varied under operating condition.
2. Description of the Related Art
The main types of conversion surfaces in present use are:
a metal surface on which ions release electrons with a certain probability after impinging. The metal surface may have a special coating to increase the probability of releasing electrons. PA1 the front surface of a microchannel plate. Actually the ions do PA1 penetrate some 10 .mu.m deep into the channels before releasing electron. In this spirit the conversion surface really has a very complex form. For the discussion that follows the front surface of the microchannel plate will be equated to the conversion surface. The penetration of ions into the channels will not be considered any more, because these few 10 .mu.m can be neglected compared to the other timing errors involved. PA1 Ions can be scattered inelastically on the mesh lines. If their path continues toward the conversion surface they may arrive at incorrect times, PA1 Ions can be scattered under large angles from the mesh lines, which also changes the velocity component toward the conversion surface. PA1 Ions can hit the mesh lines and break into pieces upon impact. These pieces can also arrive at incorrect times on the ion-electron conversion surface. PA1 All paths should start from a starting surface(12) normal to the axis of the detector. PA1 All paths should start parallel to the axis of the detector with the same velocity into the detector. PA1 All paths should be determined for exactly the same time-of-flight. Use as reference time that time which is necessary for an ion flying on the axis from the starting surface(12) to the conversion surface(3).
The probability of releasing electrons on the ion-electron conversion surface strongly depends on the velocity with which an ion hits the surface. Since the velocity is inversely proportional to the square root of the mass, the probability of detection falls off strongly for ions of higher mass.
Thus, to detect ions of high mass, it is mandatory to postaccelerate these ions before they hit the ion-electron conversion surface. Then they will release electrons with a sufficiently high probability when impinging on the surface. The detector must have a sufficiently high accelerating field in front of its conversion surface. This high postaccelerating field can be the source of timing errors.
It is usual practice to keep the timing errors small by making the postaccelerating field homogeneous. The direction and magnitude of a homogeneous electrical field is independent of location. In a detector with homogeneous electrical fields the time-of-flight from the reference plane to the ion-electron conversion surface is independent from where in the reference plane the ion is started. The time-of-flight is also independent of the location where the ion enters the postaccelerating field.
Such an electrical field can only he produced by separating the drift space of the time-of-flight mass-spectrometer from the postaccelerating field by an electrically conducting mesh. An example of such a detector can he seen in FIG. 5 of the publication by de Heer et al. (Review of Scientific Instruments, volume 62(3), page 670-677, 1991).
Ions entering the detector can also hit the lines of the mesh. As long as these ions are just removed from the ion beam, this will only cause a slight reduction in signal-output from the detector. However, there are several possibilities, that ions hitting the mesh lines cause an output-signal from the detector at incorrect times:
If, because of the above named problems, it is necessary to omit the meshes, the postaccelerating field will necessarily be inhomogeneous. This causes ions on different paths to strike the ion-electron conversion surface after different flight times.
As already mentioned, the magnitude of the time errors are a function of the distance the ion path has from the ion optical axis. The variable in this function is to be taken as the distance to the ion optical axis in the reference plane, and not on the conversion surface. In the optimum case, i.e. when the conversion surface can be tilted, the magnitude of these time errors is proportional to the square of the distance to the ion optical axis.
If this is the case, and if the flight time errors should be small, one should let the ions enter the detector only close to the ion optical axis. This means starting the ions from the reference plane only close to the ion optical axis. It does not make a difference whether ion paths are focused onto a smaller area or defocused onto a larger area: measure for the sensitivity of the detector is the size of the area in the reference plane from which ions can be started with acceptably small flight time errors into the detector.
An example of this solution to the problem can be seen in the publication of Steffens et al. (Journal of Vacuum Science and Technology, volume A3(3), page 1322-1325, 1985). FIG. 4 of the PCT-Application WO 92/19367 also demonstrates this method of solving the problem. The disadvantage of these solutions lies in the fact, that only a comparativity small volume of the detector can be used, i.e. only a small area of the reference plane can be allowed to oppose the incoming ion beam. This will reduce the sensitivity of the detector.